2-neopentylanthraquinone, processes for its preparation and methods for its use

ABSTRACT

The novel compound 2-neopentylanthraquinone which is prepared by Friedel-Crafts acylation of neopentylbenzene with phthalic anhydride and subsequent cyclization of the resulting 2-(4-neopentylbenzoyl)-benzoic acid in the presence of a strong, anhydrous acid, is a catalyst for the preparation of hydrogen peroxide.

The present invention relates to the novel compound2-neopentylanthraquinone of the formula ##STR1##

The present invention furthermore relates to the preparation of2-neopentylanthraquinone by Friedel-Crafts acylation of neopentylglycolwith phthalic anhydride and subsequent cyclization of the resulting2-(4-neopentylbenzoyl)-benzoic acid in the presence of a strong,anhydrous acid.

The present invention also relates to the use of2-neopentylanthraquinone as a catalyst in the preparation of hydrogenperoxide.

The industrially most important process for the preparation of hydrogenperoxide is the anthraquinone process. For this process,2-alkylanthraquinones are required as catalysts.

In the process, which is carried out as a cyclic process, the dissolved2-alkylanthraquinone is hydrogenated catalytically to the 2-alkylanthrahydroquinone, which, after removal of the hydrogenation catalyst, isoxidized back to the 2-alkylanthraquinone with oxygen. In this secondstep, hydrogen peroxide is formed and it is extracted from the reactionsolution with water. The 2-alkylanthraquinone formed again is recycledinto the recirculation. The process is described in more detail in, forexample, Ullmann, Vol. 17, page 691 et seq., (4th Edition, Weinheim, NewYork, 1979).

The patent literature proposes various 2-alkylanthraquinones ascatalysts in this process, for example 2-methyl-, 2-ethyl-, and2-propylanthraquinone (U.S. Pat. No. 2,158,525), 2-isopropyl-,2-sec-butyl- and 2-tert-butylanthraquinone (U.S. Pat. No. 2,935,381) and2-amylanthraquinone (German Published Application DAS 1,112,051 and U.S.Pat. No. 3,041,143).

2-Amylanthraquinone is a mixture of roughly equal amounts of2-tert-pentylanthraquinone and 2-(3-methylbut-2-yl)-anthraquinone. Thismixture results from the first stage of the anthraquinone synthesis, ie.the Friedel-Crafts acylation of tert-pentylbenzene with phthalicanhydride, in which about half the tert-pentyl groups are isomerized tosec-pentyl groups.

2-Amylanthraquinone mixtures having a high content oftert-amylanthraquinone have proven particularly advantageous for thepreparation of hydrogen peroxide (German Laid-Open Application DOS2,013,299).

Although the isomerization of the tert-amyl group under the synthesisconditions can be partially suppressed, additional measures are requiredfor this purpose (German Patent 2,720,294). Some of these additionalmeasures require expensive technical apparatuses and give unsatisfactoryyields.

High stability of the 2-alkylanthraquinone is important with regard to along life of the 2-alkylanthraquinone/2-alkylanthrahydroquinone system.Furthermore, high solubility of the2-alkylanthraquinone/2-alkylanthrahydroquinone system in the solventsystems used in the H₂ O₂ preparation is advantageous for a goodspace-time yield of the hydrogen peroxide process.

However, the amylanthraquinone mixtures have the disadvantage that onlythe tert-amyl compound has good stability whereas2-sec-isoamylanthraquinone undergoes oxidation under the reactionconditions of the H₂ O₂ synthesis.

It is an object of the present invention to provide a catalyst for thehydrogen peroxide synthesis, which catalyst has high solubility in thequinone and hydroquinone forms in the reaction solution, has goodstability under the conditions of the H₂ O₂ synthesis and furthermorecan be prepared in a simple and economical manner.

We have found that this object is achieved by the novel compound2-neopentylanthraquinone of the formula ##STR2## Furthermore, we havefound a process for the preparation of 2-neopentylanthraquinone, whereinneopentylbenzene is acylated with phthalic anhydride by a Friedel-Craftsreaction to give 2-(4-neopentylbenzoyl)-benzoic acid and the latter iscyclized in the presence of a strong anhydrous acid to give2-neopentylanthraquinone. We have also found that2-neopentylanthraquinone is a catalyst for the preparation of hydrogenperoxide.

2-Neopentylanthraquinone is obtained from neopentylbenzene and phthalicanhydride in good yields without isomerization or degradation of theneopentyl group taking place.

In the synthesis of other 2-alkylanthraquinones, degradation of thealkyl group under the synthesis conditions oftens leads to the formationof undesirable byproducts, for example anthraquinone,2-methylanthraquinone and 2-isopropylanthraquinone, which presentproblems in the H₂ O₂ synthesis and therefore have to be separated offin expensive purification steps.

Since no such byproducts are formed in the synthesis of2-neopentylanthraquinone, this expensive purification can be dispensedwith.

In the novel process, neopentylbenzene (1) is acylated in a first stepwith phthalic anhydride (2) under Friedel-Crafts conditions to give2-(4-neopentylbenzoyl)-benzoic acid (3). ##STR3##

Neopentylbenzene is a known compound and is described in J. Chem. Soc.(C) 18 (1969), 2505-2506.

The conventional compounds, such as FeCl₃, BF₃, ZnCl₂ and TiCl₄, can beused as Friedel-Crafts catalysts for this reaction. Aluminum halides,preferably aluminum bromide and in particular aluminum chloride, areparticularly suitable. Mixtures of these catalysts can also be used.

The amounts of catalyst usually used for Friedel-Crafts reactions canalso be used in the novel process, 2 moles of catalyst per mole ofphthalic anhydride advantageously being employed. However, it is alsopossible to use an excess of catalyst, for example from 2.1 to 3.5 molesof catalyst per mole of phthalic anhydride.

Advantageously stoichiometric amounts of neopentylbenzene and phthalicanhydride are used. It is however also possible to employ one of the twocomponents in an excess amount relative to the other. For example, from1 to 1.5 moles of neopentylbenzene can be used per mole of phthalicanhydride or vice versa.

The reaction is carried out in the absence or, advantageously, in thepresence of a solvent, suitable solvents being, for example,chlorobenzene, dichlorobenzene, trichlorobenzene, 1,2-dichloroethane,carbon disulfide, nitromethane and nitrobenzene. The amount of solventis variable and in general from 200 to 1,000 g of solvent can be usedper mole of neopentylbenzene.

The reaction is advantageously carried out by reacting the startingmaterials at from -20° to 100° C., preferably from 0° to 60° C., inparticular from 10° to 40° C., under superatmospheric or reducedpressure, but preferably under atmospheric pressure.

Advantageously, the phthalic anhydride is initially taken together withthe solvent, and the Friedel-Crafts catalyst and then theneopentylbenzene are added, although the converse sequence is alsopossible.

Working up of the reaction mixture and isolation of the product isgenerally carried out in a conventional manner by pouring the reactionmixture onto water and/or ice, separating off the aqueous phase and thenisolating the 2-(4-neopentylbenzoyl)-benzoic acid from the organicphase.

The 2-(4-neopentylbenzoyl)-benzoic acid (3) is advantageously cyclizedin the presence of a strong, anhydrous acid to give2-neopentylanthraquinone (cf. equation). ##STR4##

The cyclization of the 2-(4-neopentylbenzoyl)-benzoic acid isadvantageously carried out in a conventional manner, for example by themethods described in Houben-Weyl, Vol. 7/3c, pages 35-39, advantageouslywith from 95 to 105% strength sulfuric acid (=5% strength oleum),preferably with 100% strength sulfuric acid.

Advantageously, the sulfuric acid is initially taken and the2-(4-neopentylbenzoyl)-benzoic acid is added to it. The amount ofsulfuric acid is in general from 0.5 to 10, preferably from 0.8 to 1.0,kg per kg of the 2-(4-neopentylbenzoyl)-benzoic acid. The reactionmixture is then stirred in general for from 1 to 10 hours at from 20 to220° C., preferably, however, at from 60° to 90° C. The reaction can becarried out under atmospheric, superatmospheric or reduced pressure.

The reaction is terminated by pouring the reaction mixture into, forexample, ice and/or water. The product can then be isolated from thereaction mixture by extraction with a suitable solvent. Examples ofsuitable solvents are aromatic hydrocarbons, such as benzene, tolueneand xylene, and chlorohydrocarbons, such as chlorobenzene,dichloromethane, dichloroethane and trichloroethane.

The solution of the 2-neopentylanthraquinone is advantageously washedneutral with dilute sodium hydroxide solution and then with water. Thesolvent is removed and the product then isolated.

However, working up can also be effected in the absence of a solvent. Inan advantageous procedure of this type, the reaction is terminated bypouring the reaction mixture into water and/or ice and the precipitatedreaction product is filtered off. After the reaction product has beenwashed neutral, the 2-neopentylanthraquinone is isolated from it,preferably by distillation under reduced pressure. This procedureresults in not only lower solvent consumption but also lessenvironmental pollution due to solvent emissions.

2-Neopentylanthraquinone is very suitable as a catalyst for thepreparation of hydrogen peroxide by the anthraquinone process (UllmannsEnzyclopadie der Technischen Chemie, Vol. 17, page 691 et seq., 4thEdition, Weinheim, 1979). However, it can also advantageously be used asa catalyst in mixtures with other anthraquinones, such as2-methylanthraquinone, 2-ethylanthraquinone, 2-propylanthraquinone,2-isopropylanthraquinone, 2-secbutylanthraquinone,2-tert-butylanthraquinone and 2-amylanthraquinone. The mixing ratio mayvary within wide limits since this depends on the one hand on the typeof 2-alkylanthraquinones admixed and on the other hand on the particularprocess conditions (temperature, solvent, etc.) in the relevant plantsfor the preparation of hydrogen peroxide.

EXAMPLE

74 g (0.5 mole) of phthalic anhydride were dissolved in 250 ml ofo-dichlorobenzene. 128 g (1.05 moles) of AlCl₃ were added a little at atime at from 15° to 20° C. 74 g (0.5 mole) of neopentylbenzene were thenadded dropwise at 20° C. in the course of 3 hours. Stirring was thencontinued for 1 hour at 40° C.

After the end of the reaction, the reacted mixture was poured onto amixture of 1 1 of water with 300 g of ice and 30 ml of concentrated H₂SO₄. The organic phase was washed neutral with dilute sodium hydroxidesolution. Thereafter, the neopentylbenzoylbenzoic acid was precipitatedfrom the aqueous phase with sulfuric acid and was dried. 140.8 g (95%)of 2-(4-neopentylbenzoyl)-benzoic acid (mp. 154.2° C.) were obtained.

90 g (0.3 mole) of this 2-(4-neopentylbenzoyl)-benzoic acid wereintroduced into 900 g of 100 percent strength sulfuric acid at 60° C.and then stirred for 4 hours at 85° C. The reaction mixture was thenpoured into a mixture of 1,500 g of water and 500 g of ice, and thestirred mixture was cooled. The precipitated crystals ofneopentylanthraquinone were filtered off, washed neutral with dilutesodium hydroxide solution and water, dried, and distilled at 240° C./0.5mbar. 75.3 g (89%) of 2-neopentylanthraquinone (mp. 93.8° C.) wereobtained.

We claim:
 1. A process for the preparation of 2-neopentylanthraquinone,wherein neopentylbenzene is acylated with phthalic anhydride by aFriedel-Crafts reaction to give 2-(4-neopentylbenzoyl)-benzoic acid andthe latter is cyclized in the presence of a strong anhydrous acid togive a composition consisting essentially of 2-neopentylanthraquinonewithout isomerization or degradation of the neopentyl group takingplace.
 2. A process as claimed in claim 1, wherein the cyclization iscarried out with from 95 to 105% strength sulfuric acid at from 20° to220° C.